Production of thiourea

ABSTRACT

A two-stage process for the production of thiourea from hydrogen sulfide and calcium cyanamide in which the hydrogen sulfide is introduced into an aqueous suspension of the calcium cyanamide in a first stage at temperatures below about 80*C. and advantageously no higher than about 40*- 60*C. under a reduced pressure of less than about 300 mm. Hg and the resulting mixture is then further reacted in a second stage at about 60*C. to 100*C. under approximately normal atmospheric pressure. The process is especially useful for the continuous production of thiourea.

United States Patent [191 Kersten et a].

[541 PRODUCTION OF THIOUREA -[75]' Inventors: Hilde Kersten, Trennfurt;Gerhard Meyer, Obernburg; Clemens Neu-' haus, Erlenbach all of GermanyNC. I

22 Filed: Nov. 12, 1970 21] Appl. No.: 89,038

30 Foreign Application Priority Data Nov. 14, 1969 Germany ..P 19 57202.2

52 us. 01. .Q. ..260/552 R 51 Int. Cl ..C07c 157/00 58 Field ofSearch...'.. ..260/552 R [56] References Cited UNITED STATES PATENTS 72,173,067 9/1939 Roblin ..260/552 [451 Mar. 27, 1973 l/l946 Lewis..260/552 7/1944 Cooper ..260/552 Primary Examiner-Leon 'ZitverAssistant Examiner-Mi chael W. Glynn Attomey-Joh'nston, Root, OKeeffe,Keil, Thompson & Shurtlefi 57 ABSTRACT A two-stage process for theproduction of thiourea from hydrogen sulfide and calcium cy'anarnide inwhich the hydrogen sulfide is introduced into an aque-' the resultingmixture is then further reacted in a second stage at about 60C. to 100C.under approximately-normal atmospheric pressure. The process isespecially useful for the continuous production of thiourea.

I 7 Claims, No Drawings PRODUCTION OF THIOUREA I Various processes areknown for the production of thiourea from free cyanamide itself or oneof its salts and an inorganic sulfide such as hydrogen sulfide, calciumsulfide, barium sulfide or ammonium sulfide. In these known processes,the reaction has been accomplished in the presence or absence of waterand in some instances with the addition of compounds which causehydrogen sulfide to be set free from the sulfides and which cause theformation of difficulty soluble alkaline earth metal salts. It isfurther known that the reaction can be carried out at normal or elevatedtemperatures as well as at normal or superatmospheric pressures.

Although calcium cyanamide is a very desirable initial reactant, ithydrolyzes easily and further reacts in alkaline medium with theformation of dicyandiamide and other undesirable by-products. In orderto repress the formation of these by-products, the reaction has beencarried out at relatively low temperatures of about C. However, whenattempting to react hydrogen sulfide with the calcium cyanamide undersuch low temperature conditions, it is necessary to proceed very slowlyon account of the high heat of reaction, e.g. such that a prohibitivelylong period of time is required for gassing the reaction mixture withhydrogen sulfide.

According to one improvement suggested in U. S. Pat. No. 2,353,997, thereaction of calcium cyanamide with hydrogen sulfide in the presence ofwater is carried out slightly'below 100C., e.g. at about 80 95C.,whereby the reaction proceeds rapidly and without the formation ofsubstantial amounts of by-products. As

, pointed out in this patent, however, this reaction is successful onlyif carried out in the presence of a large excess of the sulfide, e.g.,hydrogen sulfide, so that the unreacted portion of the sulfide must berecovered or else accepted as a complete loss. The process proposed inthe patent partly overcomes this problem by carrying out the reaction intwo stages wherein the reaction is initiated in a first stage by addingcalcium .cyanamide to I a large excess of hydrogen sulfide and thenadding fresh calcium cyanamide in a second stage for reaction with theinitial excess of hydrogen sulfide. This procedure is very cumbersomeand is not at all suitable for a commercial continuous production ofthiourea. In addition to the problems of adding calcium cyanamide andhydrogen sulfide at various adjusted rates, it is necessary tocentrifuge or filter the intermediate reaction product from the firststage before the reaction is carried out with gradually decreasingamounts of hydrogen sulfide in the second stage. Such complicationsshould be avoided incommercial practice.

A further improvement of this process has been suggested in U. S. Pat.No. 2,393,917 wherein it is pointed out that one can avoid theemployment of hydrogen sulfide in substantial stoichiometric excess aspreviously required through at least the greater part of the reaction.In order to accomplish this result, an aqueous suspension of an alkalineearth metal hydroxide is first saturated with hydrogen sulfide at atemperature of about 15 20C. in order to form an aqueous solution of analkaline earth metal hydrosulfide. This intermediate solution is thenheated to a temperature of about C. and calcium cyanamide is then addedin small portions while continuing the addition of hydrogen sulfide. Theyields of this process are relatively low, particularly when carried outwithout the addition of an alkaline earth metal hydroxide, and it isgenerally necessary to filter the reaction mixture before the reactionis completed, i.e., between successive additions of the calciumcyanamide. This procedure is means of a two-stage process which can becarried out in a relatively short period of time to achieve high yieldsand a very pure thiourea product It is also an important object of theinvention to provide a process for the continuous production of thioureain which a large excess of hydrogen sulfide can be avoided and in whichthe reaction conditions and recovery of the final product are greatlysimplified. These and other objects and advantages of the invention willbecome apparent upon consideration of the following detailed disclosure.

It has now been found, in accordance with the invention, that one canachieve a substantially improved process for the production of thioureaby introducing hydrogen sulfide in a first-stage into an aqueoussuspension of calcium cyanamide for reaction therewith at temperaturesof from about 10 to 80C. under a reduced pressure corresponding to thevapor pressure of the reaction mixture at the reaction temperatures inthis first stage up to about 300 mm. Hg and then completing the reactionin a second stage at a temperature of about 60 to C. under approximatelynor mal atmospheric pressure The phrase normal atmospheric pressure isemployed herein to define a pressure of approximately 1 atmosphere,i.e., as exists under normal atmospheric conditions. One can likewiserefer to normal temperature or room temperature as defining thattemperature which normally occurs under laboratory or factoryconditions. These so-called normal reaction conditions do not excludethe possibility of working under slightly elevated temperatures andpressures.

In carrying out the reaction according to the invention, it ispreferable to introduce the bulk of the hydrogen sulfide into the firststage at a temperature of about 20 to 60C., i.e., so that thetemperature of this first stage does not substantially exceed 60C. Forexample, it has been found particularly advantageous toinitially'introduce the hydrogen sulfide into an aqueous suspension ofcalcium cyanamide at room temperature and then, after the reactionmixture attains a temperature of about 40C., to further introduce theremaining portion of hydrogen sulfide at temperatures in the range ofabout 40-60C. The hydrogen sulfide is introduced continuously, and onecan proceed in such a manner that the hydrogen sulfide is firstintroduced into the aqueous calcium cyanamide suspension at roomtemperature and the temperature of the reaction mixture is thenpermitted to rise continuously to a maxmore reaction periods so as togradually increase the temperature throughout the first stage.

It is especially preferred to employ the hydrogen sulfide in liquidform. However, good results have also been achieved .with gaseoushydrogen sulfide. even though the rate at which it can be introducedinto the aqueous reaction mixture is somewhatslower. Liquid hydrogensulfide is of course especially preferred in a continuous process.

The time taken for introducing the hydrogen sulfide issignificant incarrying out the process of the invention. The yields vof thiourea aregenerally higher, the more quickly the amounts of hydrogen sulfide aresupplied for they quantitative reaction of the available calciumcyanamide. If the hydrogen sulfide is introduced too slowly, the yieldis correspondingly reduced. This loss of yield is especially noticeableif thereaction in the first stageis carried out predominantly between60C and 80C., i.e., if the hydrogen sulfide were to be added for themost part at such higher temperatures.

In contrast to one of the better known processes of the prior art, it isnot necessary to add hydrogen sulfide in a large excess, and the processof the present invention is therefore preferably carried out with anapproximately sthoichiometric amount of the hydrogen sulfide withreference to, the calcium cyanamide. In general, the reaction proceedsaccording to the following equation:

The process of the invention avoids the. serious disadvantages of theknown processes, including the partial hydrolysis of the calciumcyanamide due to the long periods of time required in carrying out thereaction and/or the necessarily higher reaction temperatures. It wasdiscovered that when working under a reduced pressure in the firstreaction stage, the temperature is controlled by directly leading offthe high heat of reaction by evaporation of 'water. Thus, thetemperature of the reaction mixture in the first stage duringintroduction of the hydrogen sulfide can be regulated exclusively by therate of vaporization of the'water.

It is therefore especially desirable to carry out the first stage in thepresence of a sufficient excess of water so that the heat of reactioncan be easily removed by evaporative cooling, i.e., by evaporating waterfrom the reaction mixture. Furthermore, this vaporized water can beeasily refluxed, i.e., condensed and returned to the reaction mixture inthe first stage, while maintaining the liquid reaction mixture under afree gas space at subatmospheric pressure. This subatmospheric pressurethus generally corresponds to the vapor pressure Accordingly, the molarratio of H S:CaNCN- is normally maintained below about 1.2:] andpreferably in a range of approximately 1:1 to 1.1:1. Of course, wherethe reaction is initiated by gradually introducing the hydrogen sulfideinto the aqueous suspension, there will be a substantial excess of thecalcium cyanamide until all of the hydrogen sulfide has been added.

In place of water as the aqueous medium for the reaction, it will beunderstood that one can also employ a mother liquor which containsthiourea in solution. The proportional amounts of the calcium cyanamideand water or thiourea-containing mother liquor can be varied over a widerange. However, it is preferable to work within a weight ratio of HozCaNCN of about 1:1 to 5:1. One can also work with a large excess ofwater, i.e., with much more dilute suspensions of the calciumcyanamide,- but this will normally be avoided for economical reasons.

The process according to the invention can also be carried out in thepresence of conventional catalysts or reaction accelerators, e.g., inthe presence of about 0.5

percent by weight of sulfur or similar small, catalytic amounts ofammonia (with reference to the calcium cyanamide). Such additives mayalso be introduced separately or continuously at, the same time that thehydrogen sulfide is-introduced.

of the water at the temperature in the first stage, but shouldbe-maintained below about 300 mm. Hg. Of course, other supplementalcooling means may also be employed in the first stage, and these andother minor variations should be included within the scope of theinvention.

The process can be carried out either discontinuously or continuously.However, it is especially suitable for the continuous technicalproduction of thiourea on a commercial scale because the reactionvelocity is very high. Moreover, it is not necessary to employ purereactants, and excellent results are achieved even when employing arelatively impure crude calcium cyanamide.

When carrying out the reaction as a batch or discontinuous two-stageprocess, it is preferable to first prepare the crude calcium cyanamidesuspension in water or a mother liquor containing thiourea together with0.5 percent sulfur, and to initiate the reaction by introducing hydrogensulfide at room temperature into the suspension maintained under therequired vacuum or reduced pressure. Because of the high heat ofreaction, the boiling temperature of the water under the conditions ofreduced pressure is very rapidly attained. After all of the hydrogensulfide has been introduced, e.g., in a stoichiometric amount, thereaction mixture is then preferably brought up to normal atmosphericpressure and further reacted for about 30 to 60 minutes at 60 to 100C.The best results have been achieved when this further reaction atatmospheric pressure is carried out at temperatures between about and C.After the reaction is completed, the reaction mixture can be worked upin a conventional manner in order to recover a concentrated solution ofthe thiourea.

In order to carry out a continuous production of thiourea, the aqueoussuspension of a crude calcium cyanamide as one reactant and the hydrogensulfide as the other reactant can be introduced simultaneously into asuitable reaction vessel or still maintained under reduced pressure. Thewater employed for the calcium cyanamide suspension may be a diluteaqueous solution of thiourea. The reaction vessel or still provided forthis purpose should be only partly filled with the reaction mixture,e.g., up to about two-thirds of its capacity, so as to provide a freegas space above the reaction mixture which can be maintained under avacuum. The reaction vessel is also preferably equipped with anefficient stirring or mixing means, and the gas space is preferably influid connection with a reflux condenser for the purpose of returningcondensed water back to the reaction mixture. The hydrogen sulfideshould be introduced. directly into the liquid reaction mixture, andsurprisingly very little if any hydrogen sulfide is withdrawn from thegas phase above the reaction mixture. Under steady-state conditions, theaqueous reaction mixture is essentially maintained at its boilingtemperature under the conditions of reduced temperature, and it istherefore preferable to maintain a substantially constant temperature inthis first stage of below 60C. and preferably within the range of about40 60C.

In order to achieve very high dosing rates of both the hydrogen sulfideand the aqueous calcium cyanamide suspension, it is especiallyadvantageous to work with two or several reaction vessels or stills in acascade, i.e., so that the reaction mixture flows continuously from onereaction vessel to the next in series while maintaining a uniformtemperature and reduced pressure in each reaction vessel. Similarresults can also be achieved in a single reaction tower, preferably witha continuous cocurrent flow of the reactants downwardly through thetower while withdrawing water vapor from the top of the tower. These andsimilar arrangements for continuous operation of the first reactionstage will be readily apparent to those skilled in this art.

The reaction mixture is then removed or continuously withdrawn'from thebottom of the first stage reaction vessel or the last vessel in seriesand pumped into a second reactor for completion of the reaction. Forexample, it is preferable to employ as the second stage reactor anelongated reaction tube which is easily operated under approximatelynormal atmospheric pressure. In order to prevent sedimentation or asettling out and collection of the solids in the second stage, it isdesirable to equip the reaction tubewith suitable stirring, mixing oragitating apparatus or else substantially increase the rate of flow ofthe reaction mixture through this second stage by means of narrowing thecross-section of the reaction tube, i.e., to provide a venturi effect.This second stage reaction is thus operated at approximately normalpressure and at temperatures procedure also being conventional in thisart. The wash water can be combined with the initial filtrate containingthe bulk of the thiourea in solution, or the wash water may be recycledto the first reaction stage, fresh calcium cyanamide beingsuspended andfed together with this mother liquor into the first stage. In thismanner, very good yields of thiourea can be recovered while continuouslycarrying out the two-stage reaction under relatively easily controlledconditions.

The process of the invention is further illustrated by the followingexamples, it being understood that the invention is not restricted tothese examples. All parts and percentages are by weight unless otherwiseindicated.

EXAMPLES l 6 A reaction vessel was filled to about two-thirds of itscapacity with an aqueous suspension of calcium cyanamide having thefollowing composition:

2 parts by weight H 0,

1 part by weight crude calcium cyanamide,

l percent by weight (with reference to CaNCN) conc. aqueous ammonia,

0,5 percent by weight (with reference to CaNCN) sulfur.

' The reaction vessel was then closed and evacuated for I maintain aconstant temperature. All of the hydrogen sulfide was added in everyinstance at a pressure below 300, mm. I-Ig.-At the low initial reducedpressure, the reaction mixture rapidly came to the boil and thevaporized water was continuously condensed and returned to the reactionvessel. I

After all of the hydrogen sulfide had been introduced, the reactionmixture was released to normal atmospheric pressure and heated forapproximately 30 between about 60 and 100C., prefer'ably from about 70to 80C. The cross-sectional or average retention time in this secondstage should amount to about 0.1 to 2 hours, preferably 0.5 to 1 hour.

The reaction mixture can then be continuously withdrawn from the secondstage and again worked up in a conventional manner as in thediscontinuous process. It is especially advantageous, however, to firstseparate the solids content of the reaction mixture consistingpredominately of calcium hydroxide in the form of -a so-called limesludge, e.g., by filtration or centrifuging the discharged reactionmixture in a continuous manner. The initial filtrate can be directlyevaporated for removal of water and concentration of the thiourea in thefiltrate. The filtered off solids or lime sludge is preferably washedwith water so as to substantially recoverall of the thiourea, thiswashing minutes longer at a temperature between C. and C. for completionof the reaction. The resulting mixture was worked up in the conventionalmanner for recovery of the thiourea product, i.e., by filtering off thesolids and washing them to remove any residual content of thiourea andcombining the wash waterwith the original filtrate. This filtrate wasthen concentrated by evaporation of water and a crystalline thioureaseparated therefrom in the usual manner. Theresulting thiourea wascolorless and had a melting point of C. I

The reaction conditions for each example are set forth in the followingTable I wherein t identifies the temperature of the initial aqueoussuspension of calcium cyanamide; t identifies the temperatureat whichthe bulk of the hydrogen sulfide had been introduced, i.e., near the endof the first stage of the reaction; and t identifies the temperature inthe second stage of the reaction. The yield is calculated with referenceto the initial amount of CaNCN. In all cases, the reaction was reactionmixture initiated at a temperature of 20C. The temperature was thenraised in the first stage while adding hydrogen sulfide in Examples 2-6.

TABLE I H,S Reaction First 1ntro- Temperatures Stage Example duction tI; i Pressure Yield No. Time (C.) (C.) (C.) (mm. Hg) (16) (min.) l 60 2020 7O 20 95.0 2 60 20 40 70 40 94.5 3 10 20 50 70 75 9 6.0 4 t 60 20 7070 180 92.0 5 60 20 80 80 280 '81.0 6 10 20 80 80 280 91.0

It will be noted that the yields of thiou'rea can be easily maintainedabove 90 percent by working within the conditions of the invention andpreferably such that the first stage temperature is maintained below70C. and especially below 60C. However, even if higher temperatures upto about 80C. are attained in the first stage, e.g., near the completionof the H 8 introduction, good yields arestill possible if the 11,8 isadded at a sufficiently rapid rate. Such rapid addition is preferablyachieved by supplying liquid H,S into the aqueous suspension.

The best results are obtained by working within an intermediate set ofreaction conditions with relatively rapid introduction of the H 8, andalthough the foregoing examples essentially-represent two separate batchstages, it will be apparent that the conditions are very easilyaccommodated into a continuous two-stage reaction having a continuousintroduction of the reactants and cocurrent flow through both stages,including a gradual transition of temperature-from the first stage tothe second stage. Such a continuous process is illustrated by thefollowing example.

EXAMPLE 7 about 30 liters) with the free gas space above the liquidtion, both stages of the reaction are carried out at a substantiallyconstant temperature and pressure with continuous introduction of thereactants and continuous withdrawal of the reaction mixture.

Under such steady-state conditions, there are introduced into the firststage reaction vessel about 600 kilograms per hour of a suspension ofcrude calcium cyanamide together with 0.5 percent sulfur in water (or ina dilute aqueous solution of thiourea) in a proportion by weight of H,C)':CaNCN of about 3:1 and also about 42.5 kilograms per hour of hydrogensulfide in gaseous or liquid form. (The active N-content in the crudecalcium cyanamide amounts to 23.3 percent).

F rom the bottom of the first stage reaction vessel, the is continuouslywithdrawn and pumped into a 1600 liter capacity reaction tube hydrogensulfide equipped with a mixer, this tube having a height of 3 meters anda diameter of 0.5 meters with continuous mixingover its entire height toprevent sedimentation. Suitable inserts or baffles are provided withinthe reaction tube to ensure that the reaction mixture flows therethroughin a substantially uniform strea, i.e., with approximately the same flowvelocity over its height. This second stage reactor is maintained at atemperature of to C. and under substantially atmospheric pressure in thesense that the liquid mixture is pumped from the bottom of the firststage to the top of the second stagereaction tube or tower and finally vwithdrawn from the bottom of this tower at atmospheric pressure.

The reaction mixture continuously withdrawn from the second stage isworked up by continuous filtration to separate the lime sludge from theaqueous filtrate containing the desired thiourea in solution. Theseparated sludge or solids are washed with the wash water. This washwater is combined with the initial filtrate. This mother liquorcontaining in solution all of the thiourea formed in the reaction isheated to evaporate water until the thiourea has been sufficientlyconcentrated to be separated as a crystalline precipitate. The crudeproduct is recrystallized from an aqueous solution containing gthiourea/ 1 ,000 ccm at room temperature. I

The average yield thus obtained in this continuous operation of the.process according to the invention amounted to 94 percent. The motherliquor from which the thiourea has been separated is substantiallyrecycled to the first stage of the process. Thus, excellent yields and avery pure product can be easily and effectively produced by thiscontinuous process so that it is uniquely adapted to-a large scaleindustrial production of thiourea.

The invention is hereby claimed as follows:

1. A process for the production of thiourea which comprises introducingliquid hydrogen sulfide in a first stage into an aqueous suspension ofcalcium cyanamide for reaction therewith at temperatures of from about10 to 80C. under a reduced pressure corresponding to the vapor pressureof the reaction mixture at said temperatures up to about 300 mm. Hg andthen completing the reaction in a second stage at a temperature of about60 to 100C. under approximately normal atmospheric pressure.

. 2. A process as claimed in claim 1 wherein said is added in anapproximately stoichiometric amount with reference to the calciumcyanamide.

3. A process as claimed in claim 1 wherein the reaction temperature inthe second stage I is maintained between about 70 and 80C.

4. A process as claimed in claim 1 wherein both stages are carried outcontinuously by simultaneously introducing approximately stoichiometricamounts of hydrogen sulfide and calcium cyanamide in aqueous suspensioninto a first stage reaction zone having a free gas space above theliquid reaction mixture maintained under said reduced pressure, removingthe heat of reaction from said first stage to maintain a maximumtemperature therein of not more than about 60C., con- 4 tinuouslywithdrawing said liquid reaction from first stage reaction zone andpumping it through a second ide, the solids are filtered off and washedwith water while thefiltrate is concentrated by evaporation of water,and a portion of the mother liquor obtained from at least one of saidfiltration and washing steps is recycled to said first stage reactionzone after the precipitated thiourea has been removed.

7. A continuous process as claimed in claim 4 wherein the averageretention time of the reaction mixture in said second stage reactionzone is about 0.1 to 2 hours.

2. A process as claimed in claim 1 wherein said hydrogen sulfide isadded in an approximately stoichiometric amount with reference to thecalcium cyanamide.
 3. A process as claimed in claim 1 wherein thereaction temperature in the second stage is maintained between about 70*and 80*C.
 4. A process as claimed in claim 1 wherein both stages arecarried out continuously by simultaneously introducing approximatelystoichiometric amounts of hydrogen sulfide and calcium cyanamide inaqueous suspension into a first stage reaction zone having a free gasspace above the liquid reaction mixture maintained under said reducedpressure, removing the heat of reaction from said first stage tomaintain a maximum temperature therein of not more than about 60*C.,continuously withdrawing said liquid reaction from first stage reactionzone and pumping it through a second stage reaction zone maintained attemperatures of about 60*C. to 100*C. and under approximately normalatmospheric pressure for completion of the reaction.
 5. A continuousprocess as claimed in claim 4 wherein the second stage reaction zone ismaintained at a temperature of about 70* to 80*C.
 6. A continuousprocess as claimed in claim 4 wherein a liquid reaction mixture iscontinuously withdrawn from said second stage reaction zone containingsaid thiourea in aqueous solution and a solids content consistingpredominately of calcium hydroxide, the solids are filtered off andwashed with water while the filtrate is concentrated by evaporation ofwater, and a portion of the mother liquor obtained from at least one ofsaid filtration and washing steps is recycled to said first stagereaction zone after the precipitated thiourea has been removed.
 7. Acontinuous process as claimed in claim 4 wherein the average retentiontime of the reaction mixture in said second stage reaction zone is about0.1 to 2 hours.